U.S. Pat. No. 5,477,250 issued on Dec. 19, 1995 discloses a direct printing apparatus. In the direct printing apparatus, four printing stations are disposed along a sheet conveying direction. Each printing station comprises a toner carrier retaining toner on its outer periphery, a backing electrode opposed to the toner carrier and a printing head disposed between the toner carrier and the backing electrode, the printing head having a plurality of apertures and a plurality of electrodes surrounding each aperture. The backing electrode of each printing station is electrically connected to a power source, thereby between the toner carrier and the backing electrode is formed an electric field for attracting the toner on the toner carrier and propelling it toward the backing electrode through the apertures of the printing head. Between the printing head and the backing electrode in each printing station is formed a passage for a sheet.
When an ON voltage is applied to the electrode of the printing head in the printing station, the toner attracting force due to the electric field between the toner carrier and the backing electrode propels the toner on the toner carrier through the apertures toward the backing electrode and adheres it to the sheet. When an OFF voltage is applied to the electrode of the printing head, the toner attracting force does not affect the toner on the toner carrier, whereby the toner is never propelled. Thus, when ON and OFF voltage applied to the electrode of the printing head are controlled on the basis of a desired image signal, an image corresponding to the image signal is printed on the sheet.
In the aforementioned direct printing apparatus, a distance between the printing head and the toner carrier affects the flying distance of the toner. Thus, the distance between the printing head and the toner carrier necessitates an allowance of approximately 10 .mu.m, thereby high accuracy is required. Conventionally, for example, in Japanese patent Laid-open publication 6-297753, as means for positioning the printing head and the toner carrier (developing roller) to ensure the accuracy of the position, there has been provided a spacer made of resin between the printing head and the developing roller such that the spacer comes into contact with the developing roller.
However, the aforementioned direct printing apparatus has the following disadvantage. Since the spacer comes into contact with the developing roller, the toner particles enter and accumulate in the contact portion therebetween. Thus, the heat due to the long time operation of the apparatus causes the accumulated toner particles to gradually deteriorate and melt to adhere to the surface of the spacer. Then, the adhered toner provide noise to the thin uniform layer of toner particles formed on the outer periphery of the developing roller and disturb the uniform layer, whereby noise appear on the printed image.